The present disclosure generally relates to improvements in bearing systems for use in high speed rotating machinery or turbomachines. More particularly, the invention relates to an improved bearing system having a high speed rotary shaft supported by a pair of angular contact bearings subjected to a constant axial thrust preload.
There are generally two types of turbomachines that include “open” turbomachines (e.g., propellers or windmills) and “closed” turbomachines (e.g., vehicle turbochargers). Open turbomachines operate on an infinite amount of fluid, such as water or wind. On the other hand, closed turbomachines operate on a finite quantity of fluid as it passes through a housing or casing. Vehicle turbochargers, for example, typically operate as closed turbomachines wherein a centrifugal compressor is powered by a turbine driven by the exhaust gases emitted from the vehicle engine as part of the combustion cycle. Of course, turbomachines, and more specifically turbochargers, are not necessarily limited in application to vehicles. In essence, turbomachines and turbochargers may be used in a wide variety of applications wherein it may be desired to harness energy from byproducts of the combustion process, such as the combustion gases.
More specifically, turbochargers are well known in the art for use in supplying charge air under pressure to an internal combustion engine for the purpose of increasing engine performance. Such turbochargers generally have a turbine wheel and a compressor wheel or impeller mounted on a common shaft supported by suitable bearings for high speed rotational operation. The turbine wheel is positioned within a turbine housing shaped for flow-through passage of engine exhaust gases which rotatably drive the turbine wheel at a relatively high speed. The thus-driven shaft and associated bearings are typically mounted within a so-called center housing disposed between the turbine housing and a compressor housing having the compressor impeller therein. Accordingly, the exhaust-gas driven turbine wheel rotatably drives the compressor wheel which draws in and compresses ambient air to provide pressurized charge air to the associated internal combustion engine.
Significant design and development effort has focused upon the turbocharger shaft bearings in attempts to provide reduced bearing friction losses in combination with smooth and substantially vibration-free shaft rotation in a bearing configuration that is compatible with the relatively high speed and temperature transient conditions of a turbocharger operating environment. In this regard, numerous configurations have been proposed for oil-lubricated sleeve-type journal bearings such as floating bushings mounted generally at opposite ends of the turbocharger shaft at locations generally and respectively adjacent the turbine and compressor housings. Such sleeve-type bearing systems have additionally required a separate thrust bearing typically in the form of a radial collar on the rotating turbocharger shaft to sustain axial loads during operation. However, such collar-style rotating thrust bearings have been associated with substantial friction losses.
In recent years, improved turbocharger bearing systems using improved anti-friction ball bearings have been proposed. In this regard, angular contact ball bearings have been suggested wherein a pair of angular contact ball bearing units is provided for supporting opposite ends of the high speed turbocharger shaft. See, for example, U.S. Pat. Nos. 6,739,845, 6,877,901, and 7,025,579, all of which are herein incorporated by reference. In such designs, the turbine-end and compressor-end ball bearing units are mounted within a common, generally cylindrical bearing carrier, and are respectively designed to carry thrust loads acting in opposite axial directions, namely, an inboard direction (i.e., with the thrust loads acting axially toward each other). Accordingly, the pair of angular contact bearing units provides both rotary and thrust bearing functions. At least one of the angular contact ball bearing units is further associated with a spring or tolerance ring for applying an axial outboard-directed thrust pre-load force to the associated bearing unit outer race, thereby accommodating at least some axial migration of the outer race relative to the bearing carrier in response to thermal transients and the like.
One important aspect of the bearing system in turbochargers is increasing the life of the ball bearing units. Turbochargers that use oil journal bearings are susceptible to failure when exposed to high thrust loads, harsh diesel engine applications, hot end thermal soak back that causes coking of the oil local to the oil journal bearings, and irregular oil changes. With internal combustion engines increasing in performance and efficiency, an improvement in the durability and efficiency of the turbocharger unit is becoming a requirement. To address this requirement, ball bearings have become a preferable use in turbochargers to increase durability. A turbocharger equipped with properly sized ball bearings will oftentimes see improvements in time to boost, radial and axial load capacity, tighter rotating clearances among components (resulting in improved turbocharger efficiency), increased tolerance to oil maintenance intervals, transient operating conditions, and thermal soak back in the hot section of the turbocharger.
Thus, there exists a constant need in the art for an improved bearing system that includes a pair of axially split and generally cylindrical cartridges having respective bearing units configured to be mountable to a common axial shaft, wherein such cartridges and bearing units are subject to a mechanical or fluid-based axially inboard-directed thrust preload to increase the operating efficiency and durability of the bearing units. The present invention fulfills these needs and provides further related advantages.